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Trumpetnut

Tal 100RS...So, come on. Just how good are they?

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I suppose my eyepieces are collectively worth around 5x as much as my most expensive scope cost so I can't talk :rolleyes2:

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....... I'll try the Vixen with the TAL diagonal sometime and see if I can reverse the effect.....

I've tried this tonight. Nothing wrong with the TAL diagonal. It's performing excellently with the Vixen. Very nicly concentric diffraction rings and the light transmission seems just the same as it usually is with this scope. With the Vixen being faster than the TAL I assume that it would be a bit more sensitive to mis-collimation of the diagonal but there is none to be seen.

So the slight mis-collimation of the TAL is coming either from the focuser optical axis not being quite aligned with that of the objective lens or the objective lens needing a slight tilt adjustment. Somethings to experiment with there :smiley:

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That sounds pretty conclusive John, is there any up down left right play in the focuser, and obvious focuser adjustment screws? My guess is the focuser is more likely to go out than the objective, I don't know what you think.

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That sounds pretty conclusive John, is there any up down left right play in the focuser, and obvious focuser adjustment screws? My guess is the focuser is more likely to go out than the objective, I don't know what you think.

I'll need to give some close attention to the TAL focuser Chris. It needs a strip down and re-grease. I could enlarge the screw holes that hold it onto the tube a little which would give the scope to tilt the focuser a bit. Likewise with the objective cell I reckon.

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I'll need to give some close attention to the TAL focuser Chris. It needs a strip down and re-grease. I could enlarge the screw holes that hold it onto the tube a little which would give the scope to tilt the focuser a bit. Likewise with the objective cell I reckon.

Interesting, this wouldn't be possible on my 100R because both the focuser and objective cell are threaded and screw directly to the tube. I would really need to look into fettling the focuser assembly, which to be honest, it could do with.

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Edit to my previous post.

Before you take a drill to your Tal, would it be worth trying to rotate the focuser/ lens cell 120 or 240 degrees to see if the mounting position there makes any difference?

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Edit to my previous post.

Before you take a drill to your Tal, would it be worth trying to rotate the focuser/ lens cell 120 or 240 degrees to see if the mounting position there makes any difference?

Thanks for the suggestion Roy :smiley:

When I get some time I've a few things like this that I'm going to try, including rotating the objective and it's cell too. I need to be systematic about it so I eliminate each componant 1 step at a time. It's really a very slight amount of mis-collimation though so a small variation will probably sort it out. The diffraction rings are slightly deflected from center, not miles out :smiley:

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John, don't rotate a single lens, by all means turn the objective cell as a whole unit, but it sounds like a tiny bit of movement on the focuser would do the job, I have to be honest, I have never checked collimation on my tal

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John, don't rotate a single lens, by all means turn the objective cell as a whole unit, but it sounds like a tiny bit of movement on the focuser would do the job, I have to be honest, I have never checked collimation on my tal

I agree Jules. The 2 elements in the objective should stay the same orientation relative to each other. I'm not going to be playing with that. I have done it in the past with a 6" F/8 achromat to improve spherical aberration though.

The test I'm using is the star image at high power. The diffraction rings around the brighter stars are not quite concentric around the airey disk, they are just offset a little to one side.

I'm not going to get too obsessive about this though. The scope is already performing very well  :smiley: 

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Really want one of these on a wooden tripod to finish my TAL collection [emoji3][emoji3][emoji3]

Sent from my iPhone using Tapatalk

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Hi John

I was glad to hear the diagonal was A-ok..I really rate Tal diagonals and can't really fault them apart from the lack of compression rings..

Ref the objective do you think the elements could be slightly pinched, is to tightly held against one another? I know it's an old tip but standing the ota on it's end so it is vertical and gently tapping the outside if the cell/tube has been known to make a positive difference. Just a thought..:-)

Dave

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Hi John

I was glad to hear the diagonal was A-ok..I really rate Tal diagonals and can't really fault them apart from the lack of compression rings..

Ref the objective do you think the elements could be slightly pinched, is to tightly held against one another? I know it's an old tip but standing the ota on it's end so it is vertical and gently tapping the outside if the cell/tube has been known to make a positive difference. Just a thought..:-)

Dave

Thanks Dave :smiley:

I've used that "tapping the tube" trick in the past and it does work. It seems to settle the lens elements.

The retaining ring of the objective is just holding the lens elements in place so not too tight. I think the objective tilt needs to be adjusted slightly though. There are 3 grub screws around the lens cell that are accessable and 3 somethings that are covered by wax or past of some sort. Does anyone know what these actually do ?

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